The present invention relates to fasteners in general, and, more in particular, to fasteners that lock when set and that develop a predetermined clamp-up load while being set.
Threaded fasteners consist of an internally threaded fastener or nut and an externally threaded fastener or bolt. The nut has internal threads that thread onto external threads of the bolt. Wrenching surfaces of the nut and bolt accept torque to form a joint where the fasteners hold one or more workpieces, often called sheets, tightly together. Another name for a bolt is a threaded pin, and a nut is sometimes referred to as a collar.
Many environments in which fasteners are used require that the fasteners have extremely high integrity and strength. Use in aircraft is an example of such an environment. Fasteners must often bear loads not only along their longitudinal axis, but radially of the axis. More particularly, when fasteners join together two or more sheets and the sheets are loaded in their planes with different loads, one sheet tends to slide over the other. Fasteners passing through both sheets become loaded in shear during their resistance to this type of loading. Axial loads arise by the clamping of fastened sheets between a head of the pin on one side of the sheets and the collar on the other side of the sheets.
Fasteners quite often must respond as well in environments where they are cyclically stressed under conditions that could give rise to fatigue failure. A fastener with adequate clamp-up load on it tends to resist fatigue failure.
An obviously desirable feature of a fastener is that it does not come apart in service. Various locking devices exist that keep nuts and bolts together. One deforms the thread of the nut so that it bears in radial compression against the thread of the pin. The resistance to unthreading in this lock is purely frictional. The thread is commonly deformed at the factory in preference to the field, but field deformation has also been practiced. This type of thread lock is known as a prevailing torque thread lock.
Knowledge of the clamping load the fastener applies to a structure is also desirable. Clamp-up load correlates to the resistance of a nut to further tightening onto a bolt and against the sheets. As a clamp-up force increases, the resistance to further tightening increases, and the torque required to turn the nut increases. This fact has been used in fasteners to develop a predetermined clamp-up load.
U.S. Pat. No. 4,260,005 to Edgar Stencel discloses a self-locking collar or nut that uses external lobes to accept a wrenching tool to tighten the nut on a cooperating bolt or pin. Once a predetermined axial load exists in the joint being made, the lobes plastically deform and wrenching can no longer take place. The lobes displace material radially inward of them into and across the thread or flutes of the cooperating bolt to produce a thread lock. The thread lock results from net material deforming into and across the pin flutes. When after lobe deformation material of the nut is in the flutes of the pin, a mechanical or interference, thread lock exists. Lobe deformation is a function of setting torque applied through the wrenching tool. The advantages of the Stencel nut include free running threads prior to the forming of a thread lock, a thread lock upon reaching preload, and accurate preload.
The three lobed nut described in the Stencel patent has constant thickness walls between the lobes. In some of these nuts, upon lobe deformation, the walls between the locations of the lobes displace radially outward while the walls in the vicinity of the lobes displace radially inward, changing the shape of the nut from generally cylindrical to triangular. This triangulation enhances the thread lock. The nuts are set by a driver, such as the one described in U.S. Pat. No. 4,742,735 to Edgar Stencel. This driver has a generally triangular or deltoid shape socket. The wall material of the nuts that displaces radially outward upon lobe failure tends to interfere with the walls of the driver. This interference makes the driver "stick" to the nuts, making release of the driver from the nuts difficult.